there are always a number of such investigations in progress at the
laboratory, this ability stands Edison in good stead, for he is thus
enabled to follow, and, if necessary, correct each one step by step.
In this he is aided by the great powers of a mind that is able to free
itself from absorbed concentration on the details of one problem, and
instantly to shift over and become deeply and intelligently concentrated
in another and entirely different one. For instance, he may have been
busy for hours on chemical experiments, and be called upon suddenly to
determine some mechanical questions. The complete and easy transition
is the constant wonder of his associates, for there is no confusion
of ideas resulting from these quick changes, no hesitation or apparent
effort, but a plunge into the midst of the new subject, and an instant
acquaintance with all its details, as if he had been studying it for
hours.

A good stiff difficulty--one which may, perhaps, appear to be an
unsurmountable obstacle--only serves to make Edison cheerful, and brings
out variations of his methods in experimenting. Such an occurrence will
start him thinking, which soon gives rise to a line of suggestions for
approaching the trouble from various sides; or he will sit down and
write out a series of eliminations, additions, or changes to be worked
out and reported upon, with such variations as may suggest themselves
during their progress. It is at such times as these that his unfailing
patience and tremendous resourcefulness are in evidence. Ideas and
expedients are poured forth in a torrent, and although some of them have
temporarily appeared to the staff to be ridiculous or irrelevant, they
have frequently turned out to be the ones leading to a correct solution
of the trouble.

Edison's inexhaustible resourcefulness and fertility of ideas have
contributed largely to his great success, and have ever been a cause of
amazement to those around him. Frequently, when it would seem to others
that the extreme end of an apparently blind alley had been reached, and
that it was impossible to proceed further, he has shown that there were
several ways out of it. Examples without number could be quoted, but
one must suffice by way of illustration. During the progress of the
ore-milling work at Edison, it became desirable to carry on a certain
operation by some special machinery. He requested the proper person on
his engineering staff to think this matter up an